Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of a gaseous substrate-containing feed with an aqueous fermentation broth containing microorganisms capable of generating oxygenated hydrocarbonaceous compounds, most commonly lower alkanols such as ethanol, propanol and n-butanol. The bioconversion of carbon monoxide results in the production of oxygenated hydrocarbonaceous compounds and in many cases a lower alkanol and carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion or, as used herein, the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is, or is derived from, a synthesis gas (syngas) from the gasification of carbonaceous materials, from the reforming of natural gas and/or biogas from anaerobic digestion or from off-gas streams of various industrial methods. The gas substrate contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. For the sake of convenience, the substrate gas is referred to herein as “syngas” even though it may only contain one of carbon monoxide and hydrogen and may not be derived by the gasification of carbonaceous materials.
These anaerobic fermentation processes are suitable for continuous processes. The syngas is passed into a bioreactor the aqueous fermentation broth for the bioconversion. Off gases can be removed from the bioreactor, and aqueous broth can be withdrawn from the bioreactor for recovery of the oxygenated hydrocarbonaceous compounds or lower alkanol at a rate sufficient to maintain steady-state operation. For the anaerobic fermentations to be commercially viable, economies of scale are required. Hence, commercial scale reactors, i.e., those with liquid capacities of at least 1 million, and more often at least about 5, say, 5 to 25, million, liters would be advantageous.
In the production of lower alkanol in these commercial-scale processes, broth is withdrawn at a rate sufficient to maintain the lower alkanol at concentrations below those that adversely affect the microorganisms used for the bioconversion. Typically the concentration of the alkanol in the fermentation broth is below about 5 mass percent. Thus, a relatively dilute stream is processed to recover the lower alkanol, typically by a unit operation comprising distillation. The hydraulic retention time of the broth in the bioreactor is often less than about 3 days. Consequently, nutrients and micronutrients contained in the withdrawn broth can represent an economic loss. A commercial-scale process would thus seek to recycle water from the alkanol recovery operation. The portion of the water that can be recycled is limited as the syngas bioconversion processes produce metabolites and proteins can be present that can, in sufficient concentrations, adversely affect the microorganisms, e.g., be toxic or inhibitory to the microorganisms or affect metabolic pathways or cause a metabolic shift.
Peyton, et al., in U.S. Pat. No. 7,569,146, disclose, in connection with drawing 3, processes for obtaining potable water from still bottoms in the bioconversion of starch and sugars to ethanol. As shown in the drawing, the still bottoms is subjected to pressurized filtration, with the retentate, which contains carbohydrates, being passed to an enzymatic conversion unit operation (29a), and the enzymatic product is subjected to pressurized filtration where the sugars permeate the membrane and the retentate is sent to anaerobic digestion in CSTR reactor 32. The permeate, containing the sugars, is passed via line 29d to the ethanol fermentation. The patentees on drawing 3 caption the pressurized filtration as “MF/UF/NF”; however, they clearly state that the fermentable sugars pass through the membrane. As shown, at least two pressurized filtrations are used by Peyton, et al., where sugars are recovered and recycled to the ethanol fermentation, and the capital and operating costs offset the value of the incremental sugars being passed to the ethanol fermentation.
Although Peyton, et al., disclose the use of pressurized filtration in certain fermentation processes, they provide no disclosure or suggestion regarding reducing nutrient and micronutrient loss for anaerobic syngas fermentation processes or increasing the amount of water that can be recycled to the bioreactor in a syngas fermentation without undue adverse effect on the microorganisms or the bioconversion activity.
Accordingly, continuous processes are sought to reduce nutrient and micronutrient loss in the anaerobic bioconversion of syngas to oxygenated hydrocarbonaceous products which processes can be economically practiced on a commercial scale.